Dynamics and Structure of Negative Ions - Photoinduced double detachment on nano- and femtosecond time scales

Abstract

The goal of the presented work is to investigate double detachment of negative ions on nano- and femtosecond time scales. I have performed three types of experiments. Femtosecond time resolved pump-probe experiments have been performed on electron dynamics in the valence shell of carbon, silicon and germanium atoms. The atom under investigation was prepared by strong field photodetachment of its parent negative ion in a linearly polarized femtosecond laser pulse. The detachment process coherently populates the fine structure levels of the atomic ground state giving rise to an electronic wave packet moving in the valence shell of the atom. The wave packet is probed by strong field ionization at a time delay controlled on a femtosecond time scale. The ionized electrons are detected in an electron imaging spectrometer. The energy and angular distribution of the ionized electrons provide information on the phase of the wave packet. The results reveal a periodic spatial rearrangement of the electron cloud in the atom. The experimental procedure and detection scheme developed provide the first direct visualization of an electronic wave packet moving in the ground state of an atom. The process of rescattering in strong field detachment has also been investigated. The detached electron is accelerated in the laser field. When the E-field changes its direction the electron follows and may return and collide with the core. I have investigated the rescattering effect in negative fluorine ions and compared with previous results on bromine ions. Furthermore, strong field detachment experiments of Fð€€€ 2 were conducted and the results from molecular fluorine ions are compared to results from atomic fluorine ions. Photodetachment cross sections and doubly excited states in Kð€€€ and Csð€€€ have been investigated through double detachment. A resonant ionization scheme was used where the ion beam and laser beams were in a collinear configuration allowing for measurements of partial cross section channels individually with nearly 100% detection efficiency. A new threshold behavior has been found in photodetachment into atomic states having a large and negative polarizability. A new model was developed to explain the observed phenomena. Furthermore, two new resonances stemming from doubly excited states were found below the K(7 2P) channel opening in Kð€€€ and a set of new resonances was observed in Csð€€€ below the Cs(10 2P) channel.